TDM wireless communication systems are known to comprise a central controller that allocates a plurality of wireless communication resources, via base stations operably connected to the central controller, among a plurality of communication units. In such systems, the communication units (i.e., hand-held or in-car mobile radios) are assigned wireless communication resources in the form of a radio frequency (RF) carrier divided by time into multiple timeslots. Often, the communication units are arranged into one or more talkgroups, as known in the art. Upon receiving a request from a communication unit, the central controller allocates wireless communication resources (i.e., timeslots) for use by the communication unit and any target units intended to receive voice signals transmitted by the communication unit. Once allocated, the wireless communication resources are typically dedicated to the transmission of the voice signals, thereby preventing other communication units from using the same resources to transmit other voice signals.
Sometimes, however, it is desirable to supersede or add information to an on-going voice signal transmission, without waiting for it to finish. This feature is generally referred to as caller interrupt. For example, several police units (i.e., communication units), organized into a talkgroup, may be canvassing a geographic region looking for a suspect vehicle. While a first of the patrol units is transmitting a verbal description of the suspects as well other information to the talkgroup, a second patrol unit may spot the suspects speeding by. The second unit would like to notify the other units of the suspect's location without delay. If the second unit could interrupt the first unit, the necessary information could be transmitted quickly. The type of access control provided by current TDM wireless communication systems determines how, if at all, a communication unit can perform caller interrupt.
Currently, TDM systems provide two types of access control: queue and contention. Referring to the previous example, if queued access control is used, the talk request of the second unit will be queued until the first unit is done talking. In this case, the second unit must wait to gain access to (i.e., be allocated) the same wireless communication resources currently use by the first unit. Hence, queued access control does not provide caller interrupt since the interrupting party must wait.
If contention access control is used, the call request from the second unit is processed immediately, allowing the second unit to transmit on the same wireless communication resources as the first unit. However, this generally results in destructive interference and, in many cases, neither message is heard. Thus, contention access control does not provide reliable caller interrupt. Therefore, a need exists for a method that provides caller interrupt in TDM wireless communication systems and that overcomes the difficulties associated with prior art queuing and contention methods.